Individual B cells transcribe multiple rearranged immunoglobulin light chains in teleost fish

B cells express a unique antibody protein which comprises two pairs of immunoglobulin (Ig) heavy (H) and light (L) chains. In addition to an invariable constant (C) region, IgH and IgL chains encompass a variable (V) region mediating antigen binding. This unique region stems from Ig V(D)J gene recombination, which generates diversity by assembling these gene segments into V(H)DJ(H) and V(L)J(L) genes. To ensure that one B cell only expresses one antibody, V(H)DJ(H) rearrangement occurs only in one IgH locus (allelic exclusion), whereas V(L)J(L) rearrangement only in either the κ or λ locus (isotype exclusion). However, teleosts express multiple IgLs encoded by distinct C(L) genes. Using single-cell transcriptomics, we have demonstrated the transcription of distinct rearranged V(L)J(L)C(L) genes in single rainbow trout B cells. Our results highlight the laxity of isotype exclusion in teleosts and strongly suggest that fish B cells can produce antibodies of different specificities.